1. Field of the Invention
This invention relates to a process for purifying crude lead bullion, and more particularly relates to a process for purifying the crude lead bullion using intermediate by-products comprising sodium and calcium that must otherwise be processed or disposed of. The invention also relates to a process for treating a rough dross produced from a crude lead bullion purification step to separate out useful lead in the dross using intermediate by-products comprising sodium and calcium that must otherwise be processed or disposed of. These intermediate by-products, which are commonly referred to as a "reactive metal mixture", include a sodium filtration filter cake produced as a by-product in the manufacture of sodium, sodium sludge pellets which are obtained by squeezing sodium from a sodium filtration filter cake in a press, and sodium tankcar and storage tank heels that are obtained when the tankcars and storage tanks are cleaned.
2. Description of the Related Art
Lead bullion often contains impurities such as copper and sulfur. Copper is usually removed from the lead because the copper is a valuable resource. Sulfur is usually removed from lead because it is an undesirable contaminant.
In a typical process for removing these impurities, the bullion is tapped from a blast furnace at approximately 1200.degree. C. and then poured into a kettle. As the bullion cools, most of the copper and sulfur entrained in the lead precipitates on top of the lead in what is known as a rough dross, also known as a rough copper dross or "wet" dross, which is skimmed from the lead bullion by a crane ladle for further processing to recover the lead entrained in the dross.
The rough dross has a low copper content and a high lead content and contains, typically, 15 wt% copper sulfide (Cu.sub.2 S), 41 wt% lead sulfide (PbS), and 41 wt% metallic lead (Pb) mechanically entrained or occluded therein.
The rough dross is a heterogenous mixture of three phases, including a matte phase, a speiss phase, and an entrained lead phase. The matte phase is composed primarily of a mixture of PbS and Cu.sub.2 S, while the speiss phase usually includes entrained lead, copper arsenide (Cu.sub.3 As), copper antimonide (Cu.sub.3 Sb), and iron arsenide (Fe.sub.2 As), intermingled with an additional emulsion of very fine PbS-Cu.sub.2 S matte particles. The rough dross is processed further to recover the lead entrained in the speiss together with the PbS contained in the matte.
The rough dross is usually processed by charging it into a reverberatory furnace together with reagents such as soda ash and coke. The dross is melted in the furnace to form a separate matte phase, a speiss phase, and a pool of elemental lead. The matte and speiss phases each contain about 10-15% lead.
The processing of the dross in the reverberatory furnace liberates some of the lead entrained therein, which flows down into a molten lead pool below the matte and speiss phases. Processing the dross in this manner to remove lead from the dross is expensive and energy intensive because it involves considerable physical and mechanical handling of large quantities of hot bullion and dross, it is environmentally obnoxious because it produces hazardous fumes, and it is one that the art is desirous of eliminating from the lead processing cycle.
It has been discovered that certain alkali-metal containing materials, notably metallic sodium, improve the separation of crude lead from a dross. The improvements are realized by the strong reducing behavior of sodium, wherein residual lead sulfide is converted to lead, and by the production by the sodium of a matte phase that is fluid in nature, and which forms as a separate layer instead of a heterogenous dross. The matte permits most of the lead which would normally be entrained in the dross to enter the bullion instead.
The movement of the lead to the bullion from the matte is important because the resulting copper to lead ratio in the matte is high enough to make the matte acceptable to copper smelters, which ultimately reclaim copper from the matte. Additionally, this type of purification obviates the need for traditional reverberatory furnace processing. Further, the weight of the matte is decreased because it contains less lead, which reduces potential shipping costs for the matte.
U.S. Pat. No. 4,404,026, which is incorporated herein by reference, discloses a process for separating elemental lead from a blast furnace bullion containing a substantial amount of lead sulfide comprising the steps of forming a pool of molten lead bullion, preferably having a temperature of about 1100.degree.-1200.degree. C., transferring the bullion into a container such as a cast iron mold which is resistant to molten lead, cooling the bullion to a predetermined temperature of about 700.degree.-800.degree. C. while forming a partial matte crust over the surface of the bullion, and adding a sodium-containing reagent selected from the group consisting of metallic sodium, sodium carbonate or soda ash (Na.sub.2 CO.sub.3) and Na.sub.2 CO.sub.3 /coke to the lead bullion, with a preferred sodium-containing reagent being liquid metallic sodium in amounts of 0.5-4.0 weight percent of the bullion.
The metallic sodium reagent, which is preferably heated to just below 120.degree. C., is added to the lead bullion beneath the surface of the lead pool, so as to avoid an oxidation reaction of the reagent with air. The sodium then reacts with the lead-bearing substances, present primarily as PbS of the matte, together with a smaller amount of PbS found in the speiss, to form elemental lead, while a matte primarily comprising a mixture of sodium sulfide (Na.sub.2 S) and Cu.sub.2 S and a speiss comprising primarily a mixture of Cu.sub.3 As, Cu.sub.3 Sb and Fe.sub.2 As forms on the surface of the molten lead pool, with the elemental lead that is formed falling into the molten lead pool. Upon further gradual cooling to a temperature of about 350.degree.-400.degree. C., the matte and speiss each have a low lead content which is no more than the level of that found in the speiss and matte produced by a dross reverberatory furnace, and can be substantially less.
U.S Pat. No. 4,333,763, which is incorporated herein by reference, discloses a process for recovering lead from a previously-produced dross that contains lead sulfide and copper sulfide and has metallic lead entrained or occluded therein. Such a dross is exemplified by a rough copper dross, obtained from the rough copper drossing of lead bullion by the liquating of molten lead bullion in a conventional manner and then cooling the molten lead to a temperature of typically about 450.degree. C.
The process includes establishing a pool of molten lead in a kettle and incorporating sodium metal in the lead pool in an amount sufficient to reduce the combined lead in the lead sulfide of a rough dross to metallic lead, adding the rough dross to the molten lead, mixing together the sodium metal, molten lead, and rough dross at a temperature in the range of the melting point of metallic lead up to about 650.degree. C. to allow the sodium metal to react with the lead sulfide to reduce the combined lead of the lead sulfide to metallic lead and to produce a matte phase comprising sodium sulfide that has separated from the molten lead, with the thus-liberated metallic lead reporting in the molten lead pool and the sodium sulfide being present in the matte phase, and separating the matte phase from the lead pool.
U.S. Pat. No. 4,153,451 discloses recovering lead from a tetraethyl lead (TEL) sludge in a high temperature (900.degree.-1000.degree. C.) smelting process using a reactive metal mixture in place of sodium. This sludge is a by-product in the manufacture of TEL. In a first step of the recovery process, wet TEL sludge is dried, and the dry TEL sludge is combined with RMM to produce lead and a residue. The composition of the RMM is approximately 70% sodium and 5-30% calcium and the composition of the TEL sludge is 45-75 wt% lead.
Turning now to the reactive metal mixture, the manufacture of sodium metal includes a step of passing molten sodium through a filter to remove calcium, which is an undesirable by-product. The material remaining on the filter is a cake of sodium metal, calcium metal, and oxides of these metals with trace amounts of metal chlorides. This sodium/calcium filter cake, which is also known as a reactive metal mixture (RMM), is subsequently charged to a kettle where the RMM undergoes a recovery process to recover useful sodium. The cost of operating the sodium recovery process, however, is significant.
The RMM may also be pressed hydraulically to remove some of the residual sodium, producing "sludge pellets" which may be utilized as RMM in refining lead.
A reactive metal mixture is also produced by recovering the heels of sodium tankcars and storage tanks.
These various processes produce a large amount of RMM material, which is undesirable because if sodium is not recovered from the RMM, the disposal of RMM is difficult and expensive. RMM may be disposed of by reacting it with water to form sodium hydroxide (NaOH), but the NaOH is a very impure grade and this reaction can be hazardous.
Reusing RMM to produce sodium in electrolytic cells is also undesirable because it is an expensive process that requires high temperatures and causes rapid deterioration of equipment. This process is also undesirable due to unpredictable violent reactions.